The course of normal hemostasis and thrombosis is accompanied by proteolytic modification of fibrinogen. These proteolysis fragments derived from fibrinogen represent de novo products of the reaction of thrombin, plasmin as well as other enzymes with this protein. A major objective of this proposal is to develop specific and quantitative radioimmunoassays for measurement of these fragments as indicators of the action of thrombin and plasmin action in vivo. It is further proposed that levels of these fragments evolved in the course of hemostasis and thrombosis be correlated with in vivo perturbations in fibrinogen catabolism in experimental animal model systems. It is anticipated that the information yielded will facilitate development of improved diagnostic, therapeutic and preventive methods for thrombotic disease.